I. Field of the Invention
The present invention relates to an image pickup tube target and, more particularly, to a structure of a Se-As-Te type chalcogen photoconductive film (Saticon film) with improved sticking characteristics.
II. Description of the Prior Art
FIG. 1 shows a section of a main part of a conventional image pickup tube target and a concentration distribution of selenium, arsenic and tellurium as major constituents of a photoconductive film.
Referring to FIG. 1, a transparent conductive film 2 containing SnO.sub.2 or In.sub.2 O.sub.3 as a major constitutent is formed on the rear major surface of a disc-like transparent glass substrate 1. A very thin N-type transparent CeO.sub.2 conductive film 3 serving as a blocking layer is formed on the rear major surface of the transparent conductive film 2. A P-type photoconductive film 4 comprising a P-type Se-As-Te amorphous semiconductor film is formed on the rear major surface of the N-type transparent conductive film 3. A P-type Sb.sub.2 S.sub.3 photoconductive film 5 serving as a beam landing layer is formed on the rear major surface of the P-type photoconductive film 4. The P-type photoconductive film 4 consists of first, second and third P-type photoconductive layers 4a, 4b and 4c. The first P-type photoconductive layer 4a comprises a P-type Se-As amorphous semiconductor film having an Se concentration of 97 to 88 wt % and an As concentration of 3 to 12 wt % and is formed on the rear major surface of the N-type transparent conductive film 3 to have a thickness of 30 to 60 nm. The second P-type photoconductive layer 4b comprises a P-type Se-As-Te amorphous semiconductor film having an Se concentration of about 67 wt %, an As concentration of 3 wt %, and a Te concentration of about 30 wt % and is formed on the rear major surface of the first P-type photoconductive layer 4a to have a thickness of about 60 nm. The third P-type photoconductive layer 4c is formed on the rear major surface of the second P-type photoconductive layer 4b such that a total thickness of the multilayer film 4 is set to be about 3900 nm, for example. The third P-type photoconductive layer 4c comprises a P-type Se-As amorphous semiconductor film wherein in the Se-As concentration distribution, the As concentration continuously changes from 20 to 30 wt % to 3.+-.2 wt % over a thickness of 45.+-.20 nm which starts from the interface between the second and third P-type photoconductive layers 4b and 4c. The 3.+-.2 wt % As concentration remains unchanged as the thickness increases. The P-type photocoductive film 5 is formed on the rear major surface of the multilayer film 4. A light beam 6 is incident on the front major surface of the glass substrate 1, and a scanning electron beam 7 is supplied to the P-type photoconductive film 5.
In the image pickup tube target having the structure described above, the gradient As concentration layer as part of the third P-type photoconductive layer 4c serves as a carrier extraction layer for effectively and stably extracting carriers generated in the Te layer of the second P-type photoconductive film 4b. The gradient As concentration layer also serves to prevent the Te layer from being diffused, thereby preventing degradation of the voltage-photocurrent characteristic (V-I characteristic) forming part of evaluation criterion. The uniform 3.+-.2 wt % As concentration layer contiguous to the gradient As concentration layer serves as a capacitive layer for storage of the carriers. The P-type photoconductive layer 4c including the gradient As concentration layer and the uniform As concentration layer is the most important layer to determine quality of the electrical characteristics of the target in use.
However, when a highly luminous object or a still object is picked up with an image pickup tube having the above-mentioned target, a so-called sticking phenomenon occurs wherein a previous image sticks to the present image. The occurrence of this phenomenon is mainly dependent on the P-type photoconductive film 4. Especially, when a Saticon film is used, the sticking phenomenon largely depends on the content of highly concentrated arsenic in the third P-type photoconductive layer 4c. With the content of arsenic decreased, the sticking can be suppressed but sufficient extraction of the carriers from the Te layer cannot be sustained. As a result, a practically sufficient signal current cannot be obtained.